1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming method and an image forming apparatus, and more particularly to an image forming method and an image forming apparatus for feeding a recording medium of various types on a conveyance path extending from a registration roller pair to a fixing member, the conveyance path being shorter than a maximum length of the recording medium handled by the image forming apparatus.
2. Description of the Related Art
A related art image forming apparatus, such as a copying machine, a facsimile machine, a printer, or a multifunction printer having copying, printing, scanning, and facsimile functions, forms an electrostatic latent image on a photoconductor according to image data. The electrostatic latent image is developed with a developer (e.g., a toner) to form a toner image on the photoconductor. The toner image is transferred from the photoconductor onto an intermediate transfer member. The intermediate transfer member contacts a transfer roller to form a transfer nip therebetween. At the transfer nip, the toner image is further transferred from the intermediate transfer member onto a recording medium (e.g., a sheet) fed by a registration roller pair and nipped by the intermediate transfer member and the transfer roller. The sheet bearing the toner image is sent to a fixing nip formed by a fixing member and a pressing member contacting each other. When the sheet bearing the toner image is nipped by the fixing member and the pressing member at the fixing nip, the fixing member and the pressing member apply heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. The sheet bearing the fixed toner image is output onto an output tray.
The registration roller pair forms a registration nip to nip the sheet. At the registration nip, the rotating registration roller pair feeds the sheet toward the transfer nip. At the transfer nip, the rotating intermediate transfer member feeds the sheet toward the fixing nip. At the fixing nip, one of the rotating fixing member and the rotating pressing member feeds the sheet toward the output tray.
When a sheet having a maximum size that the image forming apparatus can handle is used, the sheet may be fed while simultaneously nipped at the registration nip, the transfer nip, and the fixing nip. In order to stably feed a sheet under such situation, an example of a related art image forming apparatus is proposed in which the linear speed Vr of the rotating registration roller pair is set to be slower than the linear speed Vc of the rotating intermediate transfer member. Further, the linear speed Vc of the rotating intermediate transfer member is set to be slower than the linear speed Vt of the rotating fixing member. Thus, a back tension can be applied to a sheet and thereby the sheet can be stably conveyed without being skewed. The linear speed Vr of the registration roller pair and the linear speed Vt of the fixing member can also be changed in accordance with the size and the slip rate of the sheet, while the above-described relationship among the linear speeds Vr, Vc, and Vt is maintained. Thus, a proper back tension can be applied to the sheet in accordance with the size and the slip rate of the sheet, and thereby formation of defective images due to an error in scaling of a toner image and/or a skew of the sheet can be suppressed.
Even when the linear speed Vr of the registration roller pair and the linear speed Vt of the fixing member are changed in accordance with the size and the slip rate of a sheet, a defective toner image having a black line (i.e., shock jitter) extending in a main scanning direction may be formed on the sheet when the thickness of the sheet is changed.
Shock jitter is formed on the second or succeeding sheet when a toner image is continuously formed on a plurality of sheets while the linear speed Vt of the fixing member is substantially faster than the linear speed Vc of the intermediate transfer member. Specifically, the foremost head of a sheet enters the fixing nip while the sheet is bent. When the linear speed Vt of the fixing member is substantially faster than the linear speed Vc of the intermediate transfer member, the fixing member feeds the sheet faster than the intermediate transfer member. As a result, the sheet, which is simultaneously nipped at the fixing nip and the transfer nip, is not bent but is stretched in a sheet conveyance direction before the tail of the sheet passes the transfer nip. The stretched sheet is conveyed at the transfer nip at the linear speed Vt of the fixing member. The sheet conveyed at the linear speed Vt of the fixing member causes the intermediate transfer member to rotate at the linear speed Vt of the fixing member. After the tail of the sheet passes the transfer nip, the intermediate transfer member is rotated by a driving force of a driver for driving the intermediate transfer member. However, the driving force is not immediately transmitted to the intermediate transfer member due to backlash of the driver and thereby the intermediate transfer member temporarily stops rotating. When a toner image is transferred from the photoconductor onto the intermediate transfer member while the intermediate transfer member temporarily stops rotating, shock jitter may be formed on the transferred toner image. The toner image having shock jitter is further transferred from the intermediate transfer member onto the second or succeeding sheet. When a thick sheet is used, the foremost head of the thick sheet enters the fixing nip while the sheet is hardly bent. Therefore, when the thick sheet is conveyed at the same linear speed ratio Vc/Vt as a plain paper sheet, the thick sheet is stretched between the transfer nip and the fixing nip quicker than the plain paper sheet. As a result, shock jitter may be formed on a toner image transferred on the second or succeeding thick sheet.
When the linear speed Vc of the intermediate transfer member is faster than the linear speed Vr of the registration roller pair, a shrunk toner image may be formed when a plain paper sheet is used. Specifically, the plain paper sheet is stretched between the registration nip and the transfer nip and thereby is conveyed at the transfer nip at the linear speed Vr of the registration roller pair instead of the linear speed Vc of the intermediate transfer member. Namely, the plain paper sheet is conveyed at the transfer nip at a speed slower than the linear speed Vc of the intermediate transfer member. As a result, a shrunk toner image is formed onto the plain paper sheet. When the linear speed Vc of the intermediate transfer member is set to be slower than the linear speed Vr of the registration roller pair to prevent formation of the shrunk toner image on the plain paper sheet, shock jitter may be formed on a toner image on the tail of a thick sheet when the thick sheet is used.